The present invention relates to a process for the utilization of electric arc furnace dust by forming a solid mass which can be used in other high temperature production processes.
In all steel production processes, waste materials are formed that contain oxidized iron and other oxidized metals such as calcium zinc, lead, chromium and cadmium. These are usually in the form of dust in the gas waste streams. This is collected in a baghouse and is referred to as baghouse dust. Dust from the electric arc furnace process is also referred to as electric arc furnace dust. This dust is difficult to process because of its fine particle size. Reintroducing the dust in to a reduction furnace to recover the iron will result in the dust once again becoming a part of the waste stream. Even though the dust contains a substantial amount of metal content, it is essentially worthless.
There are methods of storing and stabilizing the dust into piles near the production facility, but this is becoming increasingly unacceptable as environmental regulations become stricter. The dust can also be recycled and stabilized into ceramic or building materials but not necessarily on a cost-effective basis.
Of particular concern, however, is the dust created from processes typically used to recycle scrap metal. The scrap metal may come from a variety of sources such as automobiles, railroad rails, and discarded structural and sheet steel. Different mills generate a wide variation i.e., the exact composition of the metal baghouse dust. For example, electric arc furnace dust typically contains relatively large proportions of zinc, iron, and lead and smaller proportions of tin, cadmium, chromium, copper, silica, lime, and alumina. Some of the lead oxide, for example, is probably from the recycling of engine blocks which have become impregnated with lead from gasoline before the unleaded gas era and that of chromium oxide from metal scrap that had been coated with zinc chromate.
Due to the presence of the toxic oxides, the Environmental Protection Agency classifies this metal baghouse dust as a hazardous waste thus its disposal has become a major problem for steel producers. For example, to safely dispose the dust in a land fill the dust must be rendered nontoxic by removing the toxic heavy metals or by immobilizing the toxic materials in a stabilized composition for disposal or in a recycled product. Hence, there has been considerable effort to develop a treatment process for EAF dust that is economical.
U.S. Pat. No. 5,336,297 (McElroy) describes a process for treating EAF dust to make it suitable for disposal in a land fill and at the same time recovering the nonferrous metals zinc and lead for remarketing. The dust is first leached with a ferric chloride solution followed by a series of steps involving elevated temperature and pressure to form a filterable crystalline hematite. The zinc is recovered using a series of steps involving solvent extraction, stripping, and electrolysis. The lead is recovered as a precipitate upon cooling of the solution.
Another process to render EAF dust suitable for disposal in landfills is disclosed in U.S. Pat. No. 4,911,757 (Lynn et al). The process is based on the pozzolanic reaction of materials which ultimately adsorb and/or physically entrap heavy metals into a calcium-alumina-silicate matrix. The hazardous materials become virtually immobile and stable. An alkaline dry mixture of EAF dust is made by mixing it with lime kiln dust, fly ash, and hydrated line. An aqueous ferrous hydroxide and calcium sulfate solution is then added to the dry mixture and blended; during this process the EAF dust is rendered nonhazardous. Another method based on the pozzolanic reaction is provided in U.S. Pat. No. 5,245,122 (Smith). A hardened product is formed when the EAF dust is mixed with lime, ferrous sulfate, and water.
U.S. Pat. No. 5,922,261 (Ford, Jr.) provides a process for converting the EAF dust into both a fuel source and a material from which iron can be recovered. The iron containing dust is mixed with a carbon material, such as coal fines, a resin, such as a styrene polymer in a hygroscopic solvent, and a homopolymer, such as polyvinyl acetate. The resulting mixture is then compressed into some type of form, such as briquettes or pellets.
Another method for processing environmentally toxic materials in EAF dust that also provides fuel and a charging material for making molten iron or steel preproducts is described in U.S. Pat. No. 5,338,336 (Greenwalt). Petroleum coke, oxygen and EAF dust are reacted in a melter gasifier to produce reduction gas. The reduction gas is recycled to a reduction furnace that is connected to the melter gasifier where molten iron from the iron oxides is formed, and zinc, cadmium, and lead oxides are reduced, vaporized and subsequently recovered.
Using EAF dust to improve concrete or mortar materials is described in U.S. Pat. No. 5,557,031 (Al-Sugair et al.). The dust is essentially used as a partial replacement or additive for cement to enhance the cement""s workability and strength. The cement concurrently serves as a means to dispose EAF dust.
There has been considerable effort to find a means to render metal baghouse dust nonionic or to recycle the dust in some useful product. However, the economic feasibility of these processes is either unsure or unacceptable. In addition, the question still lingers as to how safe a product is that has metal baghouse dust as part of its composition, that is, are the toxic materials in the product really immobilized or rendered nontoxic. A need thus exists to use or treat metal baghouse dust in an economic and an environmentally safe way.
The present invention is premised upon the realization that metal bag house dust such as electric arc furnace dust can be solidified thereby permitting its use for a variety of different applications such as a binder and the like.
More particularly addition of an effective reducing agent to metal baghouse dust causes hardening of the metal baghouse dust thereby enabling it to be used as a binder or to be pelletized for further use.
The hardened product provides a means to add metal baghouse dust back to molten metal. Further it provides a binder which enables solidification and formation of pellets of other powders such as iron oxide and coal. Further the solidified product can be formed as a coating to encapsulate other products again enabling their use in various applications particularly applications requiring elevated temperatures. The present invention will be further appreciated in light of the following detailed description and drawings in which: